Remarkable progress has been made in an optical fiber cable as a new communication medium, and numerous cable structures have been proposed and utilized.
In the optical fiber cables, it is important to protect optical fibers assembled in the cable against an external force, since optical fibers have a drawback that when deformed upon application of an external force, they tend to be broken, and/or attenuation therethrough increases. To overcome these drawbacks of the optical fibers, several cable structures have been proposed. Among them, one cable structure has a buffer material provided around the optical fibers for protecting them against the external force or pressure. The other cable structure has a hard shell in which the optical fibers are contained.
In order to stand against tensile stress applied to the cable, a highly tensile filament, so-called a tension member, is assembled in the cable to prevent stretching of the whole cable, and thereby the tensile stress is not exerted on the optical fibers. Even if the optical fiber cable contains the tension member, it can be stretched and, therefore, the optical fibers contained in the cable are also stretched to nearly the same extent as in the cable. Since the optical fiber cable is likely to be subjected to a large tensile stress, for example, during installation of the cable, a considerably thick tension member is inevitably required, which results in an increase of the diameter and weight of the cable. Therefore, the preferred properties of the optical fiber such as light weight, a small diameter and flexibility are undesirably deteriorated.
It is known that if the residual set is present in the optical fiber, time required until the optical fiber is broken (breaking time) is shortened. This is the reason why the conventional optical fiber cables are made so that they are not easily stretched in order to avoid the application of elongation strain on the optical fibers contained in the cable.